This invention relates to a process for the biological purification of wastewater, and in particular to the problem of undesirable "bulking sludge" formation caused by the proliferation of certain microorganisms.
For a reference relating to the general field, reference is made to the monograph "Aerobic Biological Treatment of Waste Waters, Principles and Practice", Arthur W. Busch, 1971, Oligodynamics Press, Houston, Tex.
As known, bulking sludge, a "fluffy" activated sludge of a low solids weight content which impedes settling operations, is produced by the excessive growth of certain microorganisms which do not form morphologically normal activated sludge. Such bacteria are primarily filamentous bacteria and algae, as well as bacteria having thick slime jackets, and others. According to prior knowledge, the growth of such microorganisms is largely dependent on certain materials present in the wastewater, such as carbohydrates, organic acids, or heavy metals, among others. Also the lack of certain trace nutrients can have this effect. Upon the occurrence of such anomalous activated sludges, the low settling velocity of same leads to difficulties in the secondary treatment stage insofar as the biologically purified wastewater cannot be easily separated from the activated sludge. There is thus the danger that the activated sludge, by not settling at a sufficient rate, will float into the drainage ditch. This would not only place a burden on the drainage ditch, but could also result in a loss of the biologically active substance required for the biological purification.
In the case of certain wastewaters, especially those stemming from the foodstuff industry, it is not always possible to balance the supplied nutrients and the load on the activated sludge plant so as to prevent the development of the microorganisms causing the bulking sludge.
In such cases, countermeasures against the formation of bulking sludge have taken the form of either the installation of especially large secondary treatment tanks to take into account the low settling velocity of the bulking sludge, or the addition of precipitants and/or flocculants which weight down the bulking sludge and cause the same to flocculate. As for the troublesome filamentous bacteria which frequently occur in the bulking sludge, an attempt has been made to destroy these bacteria selectively by the addition of specific chemicals, such as chlorine or hydrogen peroxide.
A substantial disadvantage of merely utilizing larger secondary treatment tanks resides in the fact that microorganisms constituting the bulking sludge, which have settled or flocculated after a long residence time in the secondary treatment stage, remain present in the recycle sludge. Thus in the sedimentation stage, the recycle sludge together with identical newly formed microorganisms from the aeration stage again prevent the settling of the activated sludge in the secondary treatment stage thereby failing to eradicate the cause of the problem. Conversely, the technique employed to selectively poison the microorganisms chemically is difficult to control, and if at all effective, treats only the more sensitive types of filamentous organisms.